A-level Physics
Unit G484:
The Newtonian
World
Specific latent heat
Thermal physics

Internal energy LOs
1. Define internal energy
2. Write a statement of the first law of thermodynamics.
3. Describe two ways in which the internal energy of a body can be
increased.
4. How would you identify a body whose internal energy had increased?
5. Is it possible to increase the internal energy of a body without
changing its temperature?
6. Energy can be added to a body without raising its temperature.
How is this possible?
Thermal physics

Lesson focus
• Specific latent heat
Learning objectives
At the end of the lesson you will be able to:
• describe what is meant by the terms latent heat of fusion
and latent heat of vaporisation.
Thermal physics

Learning outcomes
All of you should be able to
• explain why a substance has two latent heats;
• give a full definition of the latent heat of fusion and of vaporisation;
• recall the equation for latent heat;
• solve straightforward problems involving latent heat.
Most of you will be able to
• solve more complex problems involving latent heat.
Thermal physics

Specific latent heat LOs
Definition
A specific latent heat of a substance is the amount of energy needed
to change the state of _____ of the substance without changing its
________________ .
This energy may be absorbed or released depending on the direction
of the change of state.
Any substance will have two specific latent heats:
• latent heat of fusion (Lf ; solid liquid)
• latent heat of vaporisation (Lv ; liquid gas) [note spelling]
Unit: __________
Thermal physics

Specific latent heat LOs
Demonstration 1:
the latent heat of vaporisation of water
We are going to find LV for water by electrical heating.
• Set up appropriate word and symbol equations to
represent the energy transfer that will take place.
electrical energy energy needed to change
=
supplied to water state of water (liquid to gas)
i.e. E = mL where, m – mass of substance
L – specific latent heat
Thermal physics

Specific latent heat
.
LOs
Demonstration 1: the latent heat of vaporisation of water
• Sketch a temperature – time graph to show the water being
heated towards boiling point.
• Describe and explain any changes to the internal energy of
the water.
• Now continue the graph as boiling starts.
• Describe and explain any further changes to the internal
energy of the water.
Thermal physics

Specific latent heat
.
LOs
Demonstration 1:
the latent heat of vaporisation of water
The accepted value for water = 2.26 x 106 J kg-1
To do
1. Comment on, and explain the difference between this value
and our experimental result.
2. Calculate the percentage difference between these values.
3. How can a more accurate determination of LV for water be
made?
Thermal physics

Changing internal energy LOs
temperature
time
To do
• On whiteboards sketch a graph showing the temperature of ice as it is
steadily heated from -10 °C to 110 °C.
• Describe and explain change to the internal energy in each section of
the graph.
Thermal physics

Changing internal energy LOs
Figure from Hyperphysics
Thermal physics